Bidentate organic phosphite ligands are characterized in that two phosphorus atoms are present in the molecule and in that one organic group (the bridging group) is bonded with both phosphorus atoms. The bidentate phosphite ligand is furthermore characterized in that each trivalent phosphorus atom is further bonded with two other monovalent organic groups or with one divalent organic group.
U.S. Pat. No. 5,235,113 describes a hydroformylation process in which a bidentate organic phosphite ligand is used in a homogeneous hydroformylation catalyst system also comprising rhodium. This patent describes a process for preparing aldehydes by hydroformylation of alkenically unsaturated organic compounds, for example 1-octene or dimerized butene, using the above catalyst system. A disadvantage of the process according to U.S. Pat. No. 5,235,113 is that the selectivity to terminal organic aldehyde compounds when starting from internally ethylenically unsaturated functional organic compounds is generally too low for a commercially attractive process. However, with some of the disclosed multidentate phosphites of U.S. Pat. No. 5,235,113, such as tetrakis(di-(2,4-di-tert-butylphenyl)phosphito)-pentaerythritol, reasonable selectivities to terminal aldehydes are achieved. A drawback of the use of these "high selectivity" ligands is that the hydroformylation activity of the catalyst system is also too low for a commercially attractive process. Increasing the catalyst activity of these catalyst systems by increasing the temperature is not possible because the ligands are thermally unstable at higher temperatures. In addition, selectivity decreases at higher temperature, because the rate of competing olefin hydrogenation reactions increases with temperature more rapidly than the rate of the hydroformylation reaction. Hydroformylation processes involving organic bidentate ligands containing two trivalent phosphorus atoms, in which the two phosphorus atoms are linked with a 2,2'-dihydroxyl-1,1'-binaphthalene bridging group, are described in the above mentioned U.S. Pat. No. 5,235,113 and in EP-B-214622, EP-A-353770, WO-A-9303839 and EP-A-556681. EP-B-213639 describes on page 38 a compound with methyl substituents on both the 3 and 3' positions of a 2,2'-dihydroxyl-1,1'-binaphthalene bridging group. However, no indication is given that the use of this class of ligands having such a bridging group would give favorable results in terms of terminal aldehyde selectivity and catalyst activity when starting from unsaturated organic compounds and especially when starting from internally unsaturated organic compounds.
WO-A-9518089 describes a process for preparing 5-formylvalerate ester starting from an internally unsaturated 3-pentenoate ester using a catalyst system comprising rhodium and a bidentate phosphite ligand, for example tetrakis(di-(2,4-di-tert-butylphenyl)phosphito)pentaerythritol.